I'd say the bloke was giving thought to going up against a whippet like rider / riders, of the same ability. Or, just a tad less. A 120lb rider, that is almost as good a rider as a 200lb, sure as hell has an advantage over the bigger bloke. To cover all the bases - make the smaller rider fit and strong. No amount of extra rider 'strength', from being bigger, could help against the 'other' bloke.
Have a squizz at most MC racers that do well. Not very many, are 'man mountains', in any MC disciplines. Even in the day's of 500 2ts, where quite a few blokes in GPs were Big Blokes, little blokes could do very well on them - see Eric Geboers (and Leisk for Aussies) amongst Thorpe, Carla, Malherbe, Martens, Van Der Venn, etc, etc. Well, that was the era of Giants. DeCoster is not a big bloke, by any means, either.